1. Field of the Invention
This invention relates to technical field of optical technology, and in particular, it relates to illumination devices, projecting devices and lighting devices.
2. Description of the Related Art
Currently, solid state light sources are widely used in general lighting, special lighting, projection and display due to their characteristics, such as long lifetime and environment friendly nature. Among these solid state light sources, the ones emitting white light show a great potential for development in the field of lighting.
There are mainly two approaches to achieve solid state white light sources. One is that a plurality of different monochromatic light sources are packaged together to generate a mixed white light. For example, red (R), green (G), blue (B) LEDs (light emitting diodes) are turned on at the same time to obtain a white light. The other one is that yellow phosphor materials are stimulated by blue light to generate yellow light, wherein the yellow converted light from the yellow phosphors and the remaining blue light which hasn't been absorbed by the yellow phosphors are mixed to obtain white light. The second approach is widely used due to its higher efficiency.
To reduce the influence of heat emitted from LED on phosphor materials, the remote phosphor technology gets applied in more and more applications, one of which is shown in FIG. 1a and FIG. 1b. In FIG. 1a, an excitation light 101 is incident on a phosphor layer 104 deposited on a minor 105, and excites the phosphor layer 104 to generate a converted light 103. Filter 102 will transmit the excitation light 101 and reflect the converted light 103. The converted light 103 becomes output light after being reflected by the filter 102.
The disadvantage of the device illustrated in FIG. 1a is that the output light contains the converted light only without the excitation light. This is because the filter 102 splits the converted light from the excitation light based on the non-overlapping nature of the spectra of the two lights. Thus, the remaining excitation light that hasn't been absorbed and is emitted together with the converted light from the phosphor layer 104, will be split away from the excitation light by the filter 102 and leave the converted light alone for output.
In FIG. 1b, the excitation light 111 is incident on a phosphor layer 114 after being transmitted by a filter 115. The filter 115 transmits an excitation light and reflects a converted light. Being excited by the excitation light, the phosphor layer 114 emits a converted light which is spilt into two parts, wherein the first part is output directly, and the second part is incident on the filter 115 then reflected by it to be output. These two parts of light and the remaining excitation light that hasn't been absorbed by the phosphor material form an output light 113.
The device illustrated in FIG. 1b can obtain a mixed light composed of the excitation light and the converted light, but the mixed light often has problems in color uniformity. Of the two components of the mixed light, the luminance distribution of the converted light is generally a Lambertian distribution, i.e. isotropic distribution, and the luminance distribution of the excitation light, which is influenced by multiple factors, such as the thickness of the phosphor layer and the luminance distribution of the excitation light 111 before impinging on the phosphor layer, is not a perfect isotropic distribution generally. These lead to a variation of proportion of the excitation light and the converted light in all propagation directions of the output light 113, and further lead to a variation of the color of the white mixed light, significantly impacting the color uniformity of the lighting source.
In fact, the phosphor material has a certain scattering effect on the excitation light. Therefore, one solution to the problem described above is to increase the amount of the phosphor material, so that all the excitation light is scattered by the phosphor material and form an output light of isotropic distribution. But one problem of this solution is that the amount of the remaining excitation light is too small, which results in a low color temperature of the final output white light. In other words, the requirement for a certain color temperature of the final output white light has confined the proportion of the excitation light and the converted light in the output light, so the usage of the phosphor materials has been limited.
In order to increase the scattering effect on the excitation light without increasing the amount of the phosphor materials, scattering materials are employed in multiple applications. For example, Chinese patent applications 201010166062.1, 200710304216.7, US patent US20090065791, U.S. Pat. Nos. 6,791,259, 6,653,765 and Korea patent KR2009054841 describe methods where scattering materials are added into a phosphor material layer to improve the color uniformity of the output light.
However, a problem of adding scattering materials is that, as the scattering effect on the excitation light is increased, a certain portion of the excitation light will be reflected directly by the phosphor material layer and transmitted through the filter 115 as energy loss. What is obvious is that, a better color uniformity of the required output light needs a larger amount of scattering materials added, which causes a greater loss of the excitation light and further a lower efficiency of the lighting device.
In short, the color uniformity of the mixed light composed of excitation light and converted light contradicts the luminous efficiency of the lighting device using the present technology.